Table 1. Registers used for outcome and covariate definition, when investigating relative effectiveness of bivalent boosters against severe COVID-19, Finland, 1 September 2022−31 August 2023.

Characteristics	Data source	Data update
Outcomes
Hospitalisation due to COVID-19	National Infectious Diseases Register	End of study (31 August 2023)
Hospitalisation due to COVID-19	Care Register for Health Care	End of study
Death due to COVID-19	Death certificates	End of study
Death in which COVID-19 was a contributing factor	Death certificates	End of study
Emergency room visit due to injury	Care Register for Health Care	End of study
Covariates
Age	Population Information System	Baseline (31 August 2022)
Region of residency	Population Information System	Baseline
Sex^a	Population Information System	Baseline
Hospitalisation between 1 September 2020 and 31 August 2022	Care Register for Health Care	Baseline
- Presence of severely immunocompromising conditions^b - Number of highly predisposing medical conditions for severe COVID-19^c - Number of moderately predisposing medical conditions for severe COVID-19^d	Care Register for Health Care	Baseline
	Care Register of Primary Health Care visits	Baseline
	Special Reimbursement Register for Medicine Expenses	30 November 2020
	Prescription Centre database	30 November 2020
Residency in a long-term care facility	Care Register for Social Care	31 December 2021
Seasonal influenza vaccination in 2022/23	Finnish Vaccination Register	End of study (31 August 2022)
Number of monovalent COVID-19 vaccinations	Finnish Vaccination Register	Baseline
Last laboratory-confirmed SARS-CoV-2 infection before the study	National Infectious Diseases Register	Baseline

SARS-CoV-2: severe acute respiratory syndrome coronavirus-2.

^a Sex was collected as binary (male/female).

^b These conditions are listed in Supplementary Table S2.

^c These conditions are listed in Supplementary Table S3.

^d These conditions are listed in Supplementary Table S4.